This thing cannot be understood because it makes no sense.
Your bf has to cut all contact with this girl and move on.
It is non-sensical that she can still dictate to him like this. She should have no control over him or you.
Old feelings die hard and cloud the judgement for long after the affection has died, but he must refuse these demands. If there is any real reason for her visit he must set the time and duration. Then stick to it.
Do you have the same powers? - if you ask him to come to you as soon as she has left will he come?
Does he realise this could affect his relationship with you?
Perhaps you could cultivate your own friends and introduce him to a new circle.

Enjoy your alone time, slob evenings are always good!
( Larry Vest One day at a time, sweet Jesus, FFS., Tue 11 May 2010, 15:38, Reply)